I don't comment on lots of the images I view either because I've nothing to say, or because I can't think of practical advice if the image needs help. But I don't post WIPs without the expectation that someone will point out to me the parts of the image that suck, and the hope that they will point me in a direction for improvement. I've had to think quite a bit about the 'constructive criticism' end of things quite a bit as a University tutor, and I certainly understand the difficulty when there is nothing absolutely nothing good I can say about a piece of work. In such instances (unless it's clear that the person was not trying to complete the exercise, but merely to irritate me say, an assigned 1500 word essay comes back with nothing but 'Sartre was a git and one of his eyes was wonky') I try to place the criticism in terms of my own understanding: "I'm not sure I understand your point here", "I can't follow your argument through this passage", "It's not clear from this that you understand the argument in question", something like that. Perhaps a technique like that would work. But then, there are just people who can't take a single solitary world of criticism. Oh, well. Let them suffer. That was a pretty scary thread, Zoe...